Hi: One thing which has come up a lot since the solar industry really took off in the last 10 years is that of corrosion and degradation of exposed metal on racks and modules.
Corrosion due to dissimilar metals, aka galvanic corrosion, is due to the natural characteristics of the metals. To get a relative sense of the magnitude of the problem, all the metals can be stacked up in table where at one end are the metals which don't corrode and the other, the ones which do. With this layout, the farther apart they are in table, the faster the corrodible metal will corrode. (This shows that zinc is a good sacrificial metal for every other metal except Magnesium). There is another effect which is at least as, or in some cases even more significant. That is the dissimilar coefficients of thermal expansion. The table below lists a bunch of common metals. The units for the numbers are, micro-meters per meter per degree Celsius. Most Expansive Zinc 30 Cadmium 30 Lead 28 Magnesium 25 Tin 23 Aluminium 22 Silver 20 Brass 19 Copper 16 Stainless Steel 16 Gold 14 Steel/Iron 11 Titanium 8 Graphite 8 Least Expansive As in the previous table, the farther apart the metals are the worse is the situation as the metals are cycled through mid winter lows to high noon summer temperatures. You can also see that steel/iron has an issue with almost every other metal. One way to deal with the galvanic corrosion is by having a zinc coating or perhaps zinc anodes up in the framework in such a place so that the zinc residue will run down the posts as the zinc oxidizes. With the thermal expansion, the use of rubber/plastic/nylon bushings will allow for thermal cycling without allowing metal fatigue and/or damage. In the case of iron, the galvanic potential between itself and aluminium is virtually non-existent. It's the thermal expansion which messes things up. Using bushings gives flexibility at the joint to allow for non-detrimental thermal expansion cycling. One other related thing to note is the the thermal coefficient of expansion for masonry is in the range of 3-4. That's a lot different than aluminium, so the "rubbery" joints help a lot in this case. JARMO _____________________________________________________________________________________ Jarmo Venalainen | Schneider Electric | Xantrex Brand | CANADA | Sales Application Engineer Phone: +604-422-2528 | Tech Support: 800-670-0707 | Mobile: +604-505-0291 Email: jarmo.venalai...@schneider-electric.com | Site: www.Xantrex.com | Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 *** Please consider the environment before printing this e-mail From: RM You <solarea...@solareagle.com> To: RE-wrenches <re-wrenches@lists.re-wrenches.org>, Date: 05/28/2015 07:21 PM Subject: [RE-wrenches] iron & aluminum Sent by: "RE-wrenches" <re-wrenches-boun...@lists.re-wrenches.org> Hi Wrenches, I have had a couple of customers want to fabricate their own PV pole mounts and racks out of iron. Aside from the galvanic corrosion and weight issues is there anything else I should caution these guys about? Any stories about failures etc? My druthers are to stay with the engineered aluminum pole mounts but when you’re dealing with someone with fabrication experience and the cost for pre-fab is high then the obvious outcome occurs. If the unit is properly treated/painted I imagine it could last many years. Is there a way to attach an anode to such a rack or would simple non conductive insulation between panels and rack be the best answer? Ron Young _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. ______________________________________________________________________
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